Electrical precipitator



Filed 'Feb. 12, 1944 3 Sheets-Sheet l Oct. 16, 1945. H. J. WHITE I2,386,827

, ELECTRICAL PRECIPITATOR Filed Feb. 12, 1944 I s Sheets-Sheet 2 EHHHHHH M magi 11 1111111111 Haw/ MAME H. J. WHITE ELECTRICAL PRECIPITATOROct. 16, 1945.

Filed Feb. 12 1944 3 Sheds-Sheet 3 \WMWVVVVVV 3mm: Hal-f7 "W I PatentedOct 16, 1945 ELECTRICAL PRECIPITATOR Harry J. White, Cambridge, Mass,assignor, by direct and mesne assignments, of one-half to ResearchCorporation, New York, N. Y., a corporation of New York and one-half toWestern Precipitation Corporation, a corporation of California LosAngcles, Calil'.,

Application February 12, 1944, Serial No. 522,115

6 Claims.

This invention relates to the electrical precipitation of suspendedparticles from gases and is particularly directed to improved collectingelectrode structures for electrical precipitators and to electricalprecipitators including the new collecting electrode structures.

Briefly, the operation of electrical precipitation consists in passinggases, containing suspended fine particles of either solid or liquid,through an electric field in which the particles become electricallycharged by attachment of electrons or ions and are then attracted to anelectrically charged member upon which the charged particles arecollected. It has been common practice to effect charging of thesuspended particles by passing them between two opposed electrodesbetween which a high potential difference is maintained, one of the twoelectrodes being a discharge electrode at which there is silent orcorona electrical discharge that ionizes the gas and causes thesuspended particles to become charged with the same'electrical sign asthe discharge electrode. This is termed charging action.

In the "single stage type of precipitator, the charged particles migrateunder the influence of the electric field between the electrodes'towardthe other electrode which is a non-discharge electrode of extendedsurface, and collect or become precipitated upon the surface of thatelectrode which is consequently termed the collecting electrode. fieldtype of precipitators, the gas containing all or a substantialproportion of the charged particles passes into a second orprecipitating field, typically maintained between opposed nondischargeelectrodes, and, under the influence of the precipitating field, thecharged particles migrate to one of the electrodes between which thefield is maintained and are deposited thereon.

The latter electrode is likewise termed a collecting electrode. Thenovel collecting electrode structures of the present invention may beadvantageously utilized as collecting electrodes in either single-stageor two-stage precipitators.

In the following description and in the appended claims the term"discharge electrode will be understood to designate an electrode thatfacilitates corona discharge therefrom, because it has a configurationthat establishes a sufllciently high potential gradient at or near itssurface to create corona discharge before there is a disruptivedischarge or spark-over. For this purpose the discharge electrodeusually takes the form of a member of small surface area, such as asmall In the "two-stage or separated or points, whereby there may becreated in the immediate vicinity thereof a sufiiclently high electricfield intensity to .cause ionization and corona discharge. The termnon-discharge electrode" will be understood to designate an electrodethat minimizes or prevents corona discharge therefrom because it has aconfiguration that establishes a sufilciently low field concentration ator near the surface to suppress corona discharge at elevated potentialslower than the voltage required for disruptive discharge or spark-over.For this purpose. a non-discharge electrode usually is one of extendedsurface area, substantially free from sharp corners or other parts ofsharp surface curvature at all portions which are located within theelectric field, so as to substantially avoid ionization of coronadischarge at that electrode.

Dust particles in the gas stream become electrically charged in theionizing field and there migrate'to the collecting electrode under theinfiuenceof the electrical forces exerted on them, and when theparticles reach and are precipitated on the electrode the electriccharges are neutralized or lost. The layer of dust particles on theelectrode is exposed ordinarily to the gas stream which tends to blowoif loose individual particles or agglomerations or particles, theerosion being a result of the gas velocity. The particles are held onthe electrode more or less securely against erosion either by electricalforces or by the physical properties of the particlesv themselves. Whenthe collecting electrode is within the range of the charging field, asin the single field type of precipitator, the field recharges andreprecipitates most of the particles that may be blown off theelectrode, but even then some particles are blown along the electrodesurface and off the outlet end of the electrode when the dust has suchphysical properties that the particles offer little resistance to gaserosion. In the two-stage type of precipitator in which there istypically no corona discharge in the second or precipitating field,material removed from the collecting electrode by erosion is notrecharged and is carried out of the precipitator by the gases.

- There are some dusts that are particularly hard to collect for thisreason, one typical example being fly ash" which is composed ofparticles that have a generally spherical shape and have very littleinherent tendency to adhere to the electrode or to each other.Characteristics such as these facilitate erosion of the precipiwhich thecritical velocity is relatively low, since.

the amount of these dusts carried into the outgoing gas stream isconsiderable and makes it difliciflt to obtain high overall collectionefllciencies at economic velocities. Obviously the critical velocity fora givendust limits the capacity of a given treater if a minimumcollection efliciency is to be obtained.

This collection difiiculty is inherent in the dust being collected andhas the efiect of reducing the capacity of a unit of a given sizeoperating at a given normal efliciency, because of the limitation ondimensions required in order to keep the gas velocity below criticalvalues. Expressed differently, the reduction in collection efficiencycaused by erosion losses increases the size of the plant and equipmentrequired to treat a given volume or gas without falling below a minimumefilciency.

In my U. S. Patent No. 2,192,250 there is described a double fieldscreen pocket precipitator including a collecting electrode structurecomprising a collecting electrode member shielded by a foraminousnon-discharging electrode member positioned between the collectingelectrode member and a discharge electrode and electrically energized toprovide a non-discharging electric field-i between the collectingelectrode member and the foraminous electrode member.

In the precipitator of the patent a certain fraction of the coronadischarge penetrates the screen electrode and passes to the shieldedcollecting electrode. By making the screen electrode more or less openthe fraction of the current thus passing to the shielded collectingelectrode can be controlled, but experience has'shown that the degree ofporosity of the screen electrode required to obtain a suitably highintensity of corona current at the collecting electrode is often sogreat that an excessive gas circulation back of the screen occurs. Inother words, the necessary compromise between the conflicting factors ofcorona current penetration and gas stream penetration is such that fulladvantage cannot be taken of the possibilities of the double fieldarrangement.

Moreover, the double field screen pocket collecting electrode of thepatent is of limited use- Iulness in the second or collecting stage of atwo-stage precipitator in conjunction with a nondlscharge typeprecipitating electrode.

These disadvantages are avoided in the collecting electrode structure ofthe present invention which provides corona discharge inside theshielded spaces of a double field screen pocket collecting electrode, inaddition to the corona discharge coming from the primary dischargeelectrode or in place of such discharge when the collecting electrode isused with non-discharging primary precipitating electrodes.

A principal object of the invention is to provide an improved electricprecipitator in which loss or collected material by erosion orredisabove which erosion loss increases persion in the gas stream issubstantially reduced or eliminated.

A further object of the invention is the provision of an improved formof collecting electrode construction for electrical precipitators.

These and other objects and advantages which will be apparent. from thefollowing description of the invention are attained by the provision ofa collecting electrode structure including extended surface membersproviding primary material collecting surfaces and defining passagesshielded by said members from the stream of gas being treated, secondarycollecting surfaces positioned within said passages, and dischargeelectrode elements carried by the members providing the primary materialcollecting surfaces and positioned to precipitate material on thesecondary collecting surfaces.

Typically the primary collecting surfaces are provided by a plurality ofvertically-extending members arranged in spaced parallel relation todefine at least one vertically-extending passageway, within which ispositioned an extended surface member spaced and electrically, insulatedfrom the vertically-extending members, said vertically-extending membersincluding discharge electrode elements, such as edges and prongs,directed toward the extended surface member within the passageway.

A typical electrical precipitator embodying the invention includes aprecipitating electrode structure anda collecting electrode structurespaced therefrom to define a gas passage therebetween and comprisingextended surface members defining a vertically-extending passageway andproviding primary material collecting surfaces and a plurality ofopenings establishing communication between the gas passage and thevertically-extending passageway, a further extended surface memberspaced and insulated from the first extended surface members andproviding secondary material collecting surfaces within thevertically-extending passageway, and discharge electrode elementscarried by the first extended surface members within theverticallyextended passageway.

The term "precipitating electrode structure" is used herein to designateelectrode means opposed to and spaced from the collecting electrodestructure and cooperating therewith to establish the electric fieldwhich causes charged particles'to precipitate on the collectingelectrode.

The invention will be more particularly described for the purpose ofillustration with reference to the accompanying drawings in which:

Fig. 1 is a sectional elevation of an electrical precipitator embodyingthe principles of the invention;

Figs. la and 1b are fragmentary details of alternative embodiments ofthe invention;

Fig. 2 is a partial plan view of the precipitator of Fig. 1 with the topcover plate removed;

Fig. 3 is a partial sectional elevation on line 3-3 of Fig. 2, and

Figs. 4 .to 8 are diagrammatic representations of illustrativeembodiments of the invention.

In Figs. 1-3, H) is a precipitator casing provided was gas inlet ll, gasoutlet l2 and a collecting hopper I3 for precipitated material.

Within the casing III are complementary precipitating and collectingelectrode structures spaced apart to define longitudinal gas passagesthrough the casing.

The precipitating electrode structures comprise discharge lectrodes ll,consisting of wires suspended from horizontal tubes l8 and maintainedtaut and in proper spaced relationship by tubes i8. Tubes 15 are carriedon I-members H which are supported on insulators l8 and insulatorbushing is contained in insulator housings 20. The precipitatingelectrode structure is energized through the insulator bushing i9.

The collecting electrode structure comprises a plurality of metalT-members 2i positioned in spaced relation and cooperating with end wallmembers 22 and partition members 28 to define vertically-extendingpassageways 26 having a plurality of vertical openings 24a. The webportions of the T-members 2i project into the passageways 2d and providedischarge electrode edges provided on the inner surfaces of the members,as shown in Fig. la, or the web of the members 2| may be notched to formtoothed edges Zlb, as shown in Fig. 2b.

The plate collecting electrode members 26 may be replaced by otherextended surface. members. such as a curtain of rods of such diameter asto be non-discharging under the conditions oi operation.

Other advantageous arrangements and constructions of the collectingelectrode structures are shown in Figs. 4 and 5, in which correspondingelements are given the same numbers as in Figs. 1-3.

therein. The members El, 22 and 23'are supported from the end walls ofcasing Ill by beam members 25 extending horizontally adjacent the upperand lower ends of members 2!, 22 and 28.

Within each of the vertical passageways 24 is positioned a collectingelectrode member comprising a plate 26 provided with rounded lateraledge members 2 by means of which the plates are suspended from tubes 28.The tubes 28 are carried on I-beams 29 which are supported frominsulators 30 and insulating bushing 3i through which the collectingelectrode plates 23 are energized.

Battles 32 aid in confining the gas flow within the interelectrodespaces.

In operation, the precipitating electrodes it are preferably maintainedat a negative polarity and collecting electrode members 26 at a positivepolarity, with the primary collecting surfaces including members 2i atground potential. However, the polarities may be reversed and any of thethree electrode elements of the precipitator may be maintained at groundpotential.

In general, it is preferable to maintain a potential difference betweenthe primary precipitating electrode and the collecting electrodestructure at about 30 to 50 kv. with a spacing of 4 inches, for example,if the primary precipitating electrode is of the discharge type, andaverage field strengths of the same order of magnitude are preferablymaintained between the secondary discharge electrode elements and thesecondary collecting members 26. If the primary precipi tating electrodeis of the non-discharge type a somewhat higher potential diflerence, say40 to 60 kv. for a. 4-inch spacing between the precipitating electrodeand the collecting electrode structure is maintained.

In operation, suspended material contained in a gas stream passedhorizontally through the apparatus is initially collected in part on theextended surfaces of the members 2i facing the precipitating electrodesI4 and in part projected through the slots 24a into passageways 28. Allor part of the material collected on the members 2i may further beeroded therefrom and thereafter carried or projected into passageways 23through the slots. The corona discharge emanating from the dischargeelements within the passageways effectively precipitates all of thesuspended material entering the passageways upon the secondarycollecting surfaces of plates 26 and maintains the deposited material onthe plates. I

The collecting electrode structures may be rapped during operation tocause the collected material to drop into a hopper or other suitablereceiver in the bottom of the apparatus.

Instead of utilizing the edge of the web of members'2l as dischargeelements, prongs Ila may be In the construction of Fig. 4 the members Hwhich define the passageways- 24 and carry discharge elements within thepassageways are shaped to provide lips to direct gas flow into thepassageways. The gas flow through the passage- ;gays may be regulated byadjustment of dampers In the construction of Fig. 5, the passageways 24are defined by extended surface members 2|" extending normal to thedirection of gas flow, the rear edges of the members 2|", which may beserrated, providing the discharge elements of the structure. eitheragainst or with the flow of gas in the main gas passage and may becurved or crimped in the cross-section shown to promote stiffness.

As is shown more particularly in Figs.-8, 7 and 8, the primarycollecting surfaces and the secondary collecting surfaces may beprovided by a series of parallel vertically-extending members ofsubstantiaily similar configuration except that alternate members areinsulated from the next adjacent members to provide two alternatingsets, the members of one of which bear discharge elements, the setsbeing differentially energized to provide a precipitating discharge fromsaid discharge elements directed toward the extended surfaces ofadjacent members.

In the construction of Figs. 6, 7 and 8, the passageways are defined byextended surface members SI, SI, 6|", respectively, providingvertically-extending collecting surfaces and bearing discharge elements62 directed generally toward the surfaces of the alternating members 63,63', 653". In each construction memberslil, 6|, 6|" are insulated frommembers 63, 63', 63" and a potential difference is maintained betweenthe two groups of electrodes as hereinbefore described. A suitableconstruction for electrical precipitators including composite collectingelectrode structures of the type shown in Figs. 8, '7 and 8, isdescribed in my application Serial No. 489,833, filed June 5, 1943.

In Figs. 6 and 8 the precipitating electrode structure is shown ascomprising discharge electrodes ll, while in Fig. 7 it consists of anextended surface electrode ll. These forms of precipitating electrodestructures are interchangeable in the various structures, the form shownin Fig. 7 being more particularly suitable for use in the second stageof a two-stage precipitator.

I claim:

1. An electrical precipitator comprising opposed precipitating andcollecting electrode structures spaced apart to provide a gas passagetherebetween, the collecting electrode structure comprising extendedsurface members defining a vertically-extending passageway within saidcollect ing electrode structure and outside saidgas passage andproviding primary material collecting surfaces opposing saidprecipitating electrode The members 2!" may be inclined extended surfaceelectrode within said passageway spaced and insulated from saidforaminous wall members.

3. In an electrical precipitator, a collecting electrode structurecomprising a plurality of vertically-extending members positioned insubstantially parallel spaced relation to form slotted walls of avertically-extending passageway and having discharge elements of smallradius of curvature projecting therefrom into said passageway, and anextended surface electrode within said passageway spaced and insulatedfrom said verticallyextending members. i

4. An electrical precipitator comprising opposed precipitating andcollecting electrode structures spaced apart to provide a gas passagetherebetween, the collecting electrode structure comprising groundedforaminous wall members defining a vertically-extending passagewaywithin said collecting electrode structure and outside said gas passageand providing a plurality of discharge electrode elements extending intosaid passa eway, an extended surface electrode within said passagewayspaced and insulated from said foraminous wall members, and means forelectrically energizing said precipitating electrode structure at onepolarity and said extended sur- .spaced face electrode at the oppositepolarity with respect to said grounded foraminous wall members.

5. In an electrical precipitator, a collecting electrode structurecomprising a plurality of parallel vertical plate members definingvertical passageways therebetween, further vertical plate membersintermediate and parallel to said first plate members and insulatedtherefrom, and discharge elements of small radius'of curvature pro-Jecting from said first plate members into said passa eways. I

.6. An electrical precipitator including opposed precipitating andcollecting electrode structures apart to provide a gas passagetherebetween, the collecting electrode structure comprising a pluralityof extended surface members spaced apart to define a plurality ofpockets open to the gas passage but shielded by said extended surfacemembers from the flow of gas therein, alternate extended surface membersbearing discharge electrode elements projecting into said pockets andbeing insulated from and maintained at an electrical potential withrespect to the remainlng extended surface members to impel chargedparticles in said pockets toward the surface of said remaining members.

HARRY J. WHITE.

